1. Field of the Invention
The present invention relates to a wireless communication apparatus such as a mobile phone with a housing which is made of two housing portions and which changes between open and closed states. More particularly, the present invention relates to an antenna apparatus for such a wireless communication apparatus.
2. Description of the Related Art
Some mobile phones are configured with a housing which is made of two housing portions and which changes between open and closed states, as shown in FIGS. 26A, 26B, 27A, and 27B. FIGS. 26A and 26B are diagrams respectively showing an open state and a closed state of a clamshell (two-fold) mobile phone according to a prior art. In this mobile phone, an upper housing 101 provided with a speaker, a display, etc. and a lower housing 102 provided with a microphone, a keyboard, etc. are coupled to each other through a hinge portion 103, and can be folded. Further, the upper housing 101 is provided with a first display device 104 and a second display device 105 which are made of liquid crystal displays or organic electro-luminescence displays. When the mobile phone is opened, the first display device 104 shows input information from an information input terminal provided to the lower housing 102, or shows received information, etc. When the mobile phone is closed, the second display device 105 shows received information, the time, etc. FIGS. 27A and 27B are diagrams respectively showing an open state and a closed state of a swivel mobile phone according to a prior art. In this mobile phone, an upper housing 111 provided with a speaker, a display, etc. and a lower housing 112 provided with a microphone, a keyboard, etc. are coupled to each other through a rotating shaft 113 that penetrates the upper housing 111 and the lower housing 112 in their thickness direction (or through an equivalent coupling mechanism). The mobile phone is opened and closed by rotation of the upper housing 111 about the rotating shaft 113. The upper housing 111 is provided with a display device 114 made of a liquid crystal display or organic electro-luminescence display. Whether the mobile phone is opened or closed, the display device 114 shows input information from an information input terminal provided to the lower housing 112, or shows received information, the time, etc.
Patent Document 1 discloses a built-in antenna in a portable wireless communication apparatus. The portable wireless communication apparatus of Patent Document 1 is provided with: a flip portion that is attached to a main body thereof to cover at least a part of the main body and that can be folded; a first metal plate in the flip portion; a second metal plate in the main body; and means for electrically connecting the first and second metal plates when closing the flip portion and for disconnecting when opening the flip portion, and the first and second metal plates are fed with radio-frequency signals at positions remote from the connecting point by certain distances.
Patent Document 2 discloses a portable wireless communication apparatus that uses circuit boards in housings as an antenna. According to the portable wireless communication apparatus of Patent Document 2, an upper housing contains an upper circuit board, a lower housing contains a lower circuit board, the upper and lower circuit boards are electrically connected to each other, and the lower housing further contains an antenna unit at top of the lower housing and contains a feeding unit for feeding the antenna unit. When the portable wireless communication apparatus is in a portable mode in which the upper housing and the lower housing are stacked upon each other, the feeding unit is connected to the antenna unit. When the portable wireless communication apparatus is in a call mode in which the total length is increased by sliding the upper housing and the lower housing, the feeding unit is connected to the upper circuit board through the antenna unit and a connecting member.
Patent Document 3 discloses a clamshell mobile phone having a plurality of antennas, and a method for changing the antennas of the mobile phone. In the mobile phone of Patent Document 3, multiple antennas are provided for each of a plurality of frequency bands, and the antennas for at least one frequency band include an antenna of a type in which its housing is used as a dipole, and another type of antenna. When an open and close detecting unit detects that a housing is closed, and when antennas for a frequency band in use include an antenna of a type in which its housing is used as a dipole, or the like, a CPU (controller) selects another antenna and changes to the another antenna. On the other hand, when the open and close detecting unit detects that the housing is opened, or when antennas for a frequency band in use do not include an antenna of a type in which its housing is used as a dipole, or the like, the CPU measures respective sensitivities when receiving through the antennas for the frequency band, selects an antenna with the highest sensitivity, and changes to the selected antenna.    (1) Patent Document 1: Japanese Patent Laid-open Publication No. H06-216621,    (2) Patent Document 2: Japanese Patent Laid-open Publication No. 2006-067361, and    (3) Patent Document 3: Japanese Patent Laid-open Publication No. 2007-274518.
Recently, as results of reducing the size of mobile phones and utilizing housings changing between open and closed states, it has become difficult to reserve a space to be provided with antenna elements in a mobile phone. Particularly, in order to maximize the electrical length of an antenna of a mobile phone changing between open and closed states, it is common practice to configure a dipole antenna made of an antenna element and a ground conductor by providing the antenna element and the ground conductor separately to a first housing portion and a second housing portion, and providing a feeding point at a position where the antenna element and the ground conductor are close to each other (i.e., a position near a coupling portion between the first and second housing portions), as described in Patent Documents 1 and 2. In such a mobile phone, when a housing of the mobile phone is opened, the antenna element and the ground conductor operate as a dipole antenna as desired. However, when the housing of the mobile phone is closed, since the antenna element in the first housing portion and the ground conductor in the second housing portion oppose to each other, current flowing through the antenna element and current flowing through the ground conductor cancel each other out, and thus the antenna element and the ground conductor cannot operate as a dipole antenna. Accordingly, in order to perform communication when a conventional mobile phone is closed, it is necessary to add a separate and dedicated antenna element to be used in the closed state, as described in Patent Document 3. Hence, a mobile phone is required that is provided with a novel antenna apparatus capable of transmitting or receiving, regardless of whether a housing of the mobile phone is opened or closed.
Additionally, antenna apparatuses have recently appeared that use the MIMO (Multi-Input Multi-Output) technique for simultaneously transmitting or receiving radio signals of multiple channels by the spatial division multiplexing, in order to increase communication capacity and thus achieve high-speed communication. An antenna apparatus for MIMO communication needs to simultaneously transmit or receive a plurality of radio signals with different directivities, polarization characteristics, etc., and thus with low correlations to each other, in order to achieve the spatial division multiplexing.